442 ZOOLOGY sect. 



Its outer layer, formed of ectoderm externally and mesoderm in- 

 ternally, is the serous rnembrane (sr. m.) : it comes to lie just 

 beneath the vitelline membrane, with which it subsequently fuses. 



The second of the embryonic membranes, the allaniois, is 

 developed as an outpushiug of the ventral wall of the mesenteron 

 at its posterior end (C, all.), and consists, therefore, of a layer of 

 splanchnic mesoderm lined by endoderm. It has at first the form 

 of a small ovoid sac having the precise anatomical relations of the 

 urinary bladder of Amphibia (Fig. 1076, A, all.) It increases rapidly 

 in size (Fig. 1077, a//.), and makes its way, backwards and to the right, 

 into the extra-embryonic coelome, between the amnion and the serous 

 membrane (Fig. 1078, C, D). Arteries pass to it from the dorsal 

 aorta, and its veins, joining with those from the yolk-sac, take the 

 blood throuufh the liver to the heart. Next, the distal end of the 

 sac spreads itself out and extends all round the embryo and yolk- 

 sac (D, all'.), fusing, as it does so, with the serous and vitelline 

 membranes, and so coming to lie immediately beneatb the shell- 

 membrane. It finally encloses the whole embryo and yolk-sac 

 together with the remains of the albumen, which has by this time 

 been largely absorbed. The allantois serves as the embryonic 

 respiratory organ, gaseous exchange readily taking place through 

 the porous shell; its cavity is an embryonic urinary bladder, 

 excretory products being discharged into it from the kidneys. 



At the end of incubation the embryo breaks the shell, usually by 

 means of a little horny elevation or carunch at the end of the beak. 

 By this time the remainder of the yolk-sac has been drawn into 

 the coelome, and the ventral body-walls have closed round it. On 

 the shell being broken, respiratory movements begin, the aperture 

 is enlarged, and the young bird is hatched and begins a free life. 



In the Ratitfe, Anseres, Gall inse, and some other Birds the young 

 when hatched are clothed with a complete covering of down or of 

 feathers, and are pj^ecocious, being able from tbe first to run about 

 and feed themselves ; such Birds are called Prmcoces or Nidifugce. 

 In the higher types, such as the Rapacious Birds, Pigeons, and 

 Passeres, the young are at first either quite naked, blind and helpless, 

 or covered w ith mere patches of soft down, so that they require to be 

 fed and kept warm by the parents ; these non-precocious forms are 

 called Altrices or Nidicolce. In many Sea-birds, such as Petrels, 

 Gulls, and Penguins, the young have a complete covering of woolly 

 down, but remain in the nest for a prolonged period, sometimes 

 until the full size is attained. 



Distribution. — The Ratitse furnish an interesting case of dis- 

 continuous distribution. Struthio occurs in Africa and South- 

 western Asia, Rhea in South America, Dromteus in Australia, 

 Casuarius in Australia, New Guinea, and some of the other Austro- 

 Malayan islands, and Apteryx in New Zealand. Thus, taking 

 recent forms onlv, each of the great Southern land-masses contains 



